This invention relates to apparatus for use in the laying and replacement of pipes underground.
It is known, for example, from British Patent Specification No. 2113795 to replace existing underground pipework by driving an expander into an existing pipeline with sufficient force to deform the pipe outwardly so that it is compressed into the surrounding ground by feeding a new pipe into the space formerly occupied by the existing pipeline.
An entirely new pipeline can be installed by the same method except that in this case the expander compresses the soil to form a passage therein for the pipe which is to be inserted.
In either case, the expander should be driven with sufficient force to ensure that the existing pipe or soil can be deformed radially outwardly and compressed into the surrounding material which may be concrete or soil.
A known apparatus for performing the above-described method of pipe replacement comprises a conical or frusto-conical member adapted to engage the end of an existing pipe with its smaller end having a smaller diameter than the internal diameter of the pipe and its larger end having a diameter which is at least as great as the external diameter of the pipe, a jacket secured to the larger end of the conical or frusto-conical member and means for driving the said member into the pipe to cause the latter to be forced radially outwards and compressed into the surrounding ground by the forward movement of the conical or frusto-conical member and jacket.
The other end of the jacket was provided with a locking ring which was screwed into the jacket, or to an interconnecting spacer member, and the leading end of a replacement pipe was secured to the locking ring by means of co-operating screw-threads.
The drive means took the form of a pneumatic percussive device referred to in the art as an impact mole. Such devices are driven through the ground by a series of impacts caused by a piston which is reciprocated within the mole casing. The following pipe is therefore dragged through the bore formed by the mole and is subjected to a succession of impact stresses as the piston repeatedly strikes the mole casing. These stresses can lead to weaknesses in and possible fracture of the pipe.